1. Field of the Invention
The present invention relates to a local oscillator without a frequency divider, and more particularly, to a local oscillator generating I and Q signals having a local oscillation frequency using a quadrature voltage controlled oscillator, a poly-phase filter, and a single side band (SSB) mixer.
2. Description of the Related Art
In modern communication technology, most amplitude and phase/frequency modulation techniques having In-phase and Quadrature-phase signal components I and Q require I/Q frequency modulation, which requires the phase of a radio frequency (RF) signal or a local oscillator (LO) signal to be modulated by 90 degrees.
In this case, the amplitude of the In-phase signal is not exactly the same as the amplitude of the quadrature-phase signal, and the phase difference between the in-phase and quadrature-phase signals is not exactly 90 degrees, either. This is referred to as an I/Q mismatch, and affects the quality of the received signal.
FIGS. 1A to 1C illustrate different structures of a conventional local oscillator that generates I/Q signal frequency using a voltage controlled oscillator and a frequency divider.
Referring to (a) of FIG. 1A, a signal having a frequency 2ω corresponding to twice a local oscillation frequency ω is generated by a differential voltage controlled oscillator 101, and the frequency of the differential voltage controlled oscillator 101 is divided in half by a frequency divider 102 to output a quadrature local oscillation frequency.
Graph (b) of FIG. 1A illustrates frequencies of output signals of the differential voltage controlled oscillator 101, a frequency divider 102, and buffer amplifiers 103 and 104. The differential voltage controlled oscillator 101 outputs the frequency 2ω to the frequency divider 102, and the frequency divider 102 outputs the local oscillation frequency ω. The buffer amplifiers 103 and 104 output a signal having the local oscillation frequency ω generated by the frequency divider 102 without any change in phase.
According to this approach, since the differential voltage controlled oscillator 101 must oscillate at the frequency 2ω and this approach include the frequency divider 102, power consumption disadvantageously increases. In addition, as the differential voltage controlled oscillator 101 operates at a high frequency of twice the local oscillation frequency, the frequency divider 102 must also operate at a fast speed.
Referring to (a) of FIG. 1B, a signal having a frequency 2ω/3 is generated by a differential voltage controlled oscillator 111, and the frequency of the differential voltage controlled oscillator 111 is divided in half by a frequency divider 112.
The resulting two signals having the frequencies 2ω/3 and ω/3 are then input to mixers 113 and 114 to output a signal having the frequency ω/3 and a signal having the frequency ω.
Graph (b) of FIG. 1B illustrates frequencies of output signals of the differential voltage controlled oscillator 111, the frequency divider 112, and buffer amplifiers 115 and 116. The differential voltage controlled oscillator 111 outputs a frequency 2ω/3 to the frequency divider 112, and the frequency divider 112 outputs a frequency ω/3.
These two frequencies are subjected to mixers 113 and 114 to output a signal having the frequency ω/3 and a signal having the frequency ω. Signals having the same frequencies ω/3 and ω as the mixers 113 and 114 are output from the buffer amplifiers 115 and 116 without any change in phase.
According to this approach, quadrature-phase signals of the output stage become cos(ωt)+cos(ωt/3) and sin(ωt)+sin(ωt/3), so that a frequency corresponding to ωt/3 remains. Such a frequency is referred to as an image frequency, and must be removed in order to output a clear and accurate signal. Therefore, when the approach of FIG. 1B is employed, a filter for removing the image frequency must be additionally disposed at an output where the frequency is converted.
Block diagram (a) of FIG. 1C corresponds to a case in which the differential voltage controlled oscillator of FIG. 1B is replaced by a quadrature voltage controlled oscillator 121, and the mixers 113 and 114 of FIG. 1B are replaced by single side band (SSB) mixers 124 and 125, which employ a single sideband frequency to cancel off the image signal so that accurate I and Q signals may be output.
Graph (b) of FIG. 1C illustrates frequencies of output signals of the quadrature voltage controlled oscillator 121, the frequency divider 123, and buffer amplifiers 126 and 127 of an output stage. The quadrature voltage controlled oscillator 121 outputs a frequency 2ω/3 to the frequency divider 123, and the frequency divider 123 outputs a frequency ω/3.
The two frequencies are subjected to SSB mixers 124 and 125 to output a signal having a frequency (ω/3 and a signal having a frequency ω. Signals having the same frequencies ω/3 and ω output by the SSB mixers 124 and 125 are output from the buffer amplifiers 126 and 127 of the output stage without any change in phase. In this case, the SSB mixers 124 and 125 are employed to reduce the magnitude of the signal having the image frequency ω/3.
However, since the configuration of FIG. 1C, like the configuration of FIG. 1B, includes the frequency divider, the frequency divider may malfunction at high frequency. In addition, the frequency of the voltage controlled oscillator for generating the quadrature-phase signal is as high as several GHz, so that accuracy of the I and Q signals is reduced.